Adhesive composition and adhered structure which thermally peeled with ease

ABSTRACT

An adhesive composition having (i) a tackifying polymer and (ii) a crystalline polymer, such that the content of the tackifying polymer is from 55 to 95 wt. % based on the whole weight of the composition, and the content of the crystalline polymer is from 4 to 40 wt. % based on the whole weight of the composition, where the said crystalline polymer comprises polycaprolactone, and the tackifying polymer comprises a polymer that is compatible with said polycaprolactone when the composition is heated to the melting point of the polycaprolactone and is crosslinked.

FIELD OF THE INVENTION

[0001] The present invention relates to an adhesive composition havingthermally easy-peeling properties, which comprises a tackifying polymerand polycaprolactone as a crystalline polymer. In particular, thepresent invention relates to the improvement of an adhesive compositionwhich will be in an easily peelable condition when it is heated to atemperature higher than the melting point of the crystalline polymer onany desired occasion after it is adhered to an adherent, so that thepeel strength is decreased to a value smaller than that prior toheating. Such an adhesive composition is particularly useful as anadhesive used in an adhesive layer of an adhesive sheet.

BACKGROUND OF THE INVENTION

[0002] Some publications disclose adhesive compositions which controlthe adhesion properties as desired by the use of a tackifying polymerhaving pressure-sensitive adhesion properties, and a crystalline polymerin combination.

[0003] For example, U.S. Pat. No. 5,192,612 (=JP-B-3021646) discloses apressure-sensitive composition comprising a pressure-sensitive adhesivebase resin (a tackifying polymer such as an acrylic polymer), adetackifying resin and a detackifying particulate. A specific preferableexample of the detackifying resin is substantially linearpolycaprolactone having a molecular weight of about 3,000 to about342,000. Polycaprolactone is a crystalline polymer which exhibitsnon-tackifying properties at room temperature (about 15 to 30° C.)

[0004] The above-described pressure-sensitive adhesive can be adhered toan adherent by pressing the adhesive against the adherent. The abovedetackifying resin and the detackifying particles effectively reducetack on the surface of the adhesive at room temperature and improvesrepeelability. The repeelability means that an adhesive material can beadhered, peeled, readhered and repeeled, and the positioning or theposition-adjustment is easy prior to the final adhesion. In this artfield, the repeeling properties are also known as “repositionability”,“slidability” (ease of sliding) or the like. The amount of the pressuresensitive adhesive base resin contained in the above pressure sensitiveadhesive is preferably from 55 to 98 wt. %, while that of thedetackifying resin is preferably from 1 to 30 wt. %, based on the entireweight of the adhesive. However, in the above US patent, it is notsupposed that the adhesive is peeled off during or after the use, oncethe adhesive is finally adhered to the adherent.

[0005] The pressure sensitive adhesive disclosed in U.S. Pat. No.5,412,035 (=JP-A-6-510548) is a pressure sensitive adhesive compositionwhich becomes pressure sensitive at least at one temperature in therange between 20° C. and 40° C., and comprises (1) at least 50 wt. % ofa polymeric pressure-sensitive adhesive component and (2) a crystallinepolymer in an amount of larger than 0 wt. % and less than 50 wt. %, eachbased on the weight of the whole solid components. The crystallinepolymer is usually non-tacky at room temperature and intimately mixedwith the polymeric pressure-sensitive adhesive component.

[0006] The melting point Ta (° C.) of the crystalline polymer, which ismeasured in the composition, is lower than the melting point Tm (° C.)of the crystalline polymer as such, and the difference Tm−Ta ispreferably from 1° C. to 9° C. In the above US patent, Tm is preferablyfrom 20° C. to 102° C. The disclosed adhesive composition is an adhesivehaving thermal repeelability, that is, a peel strength P2 (g/cm) at acertain temperature higher than Ta is smaller than a peel strength P1(g/cm) at a certain temperature lower than Ta. Such an adhesive may becalled a “thermal-peeling-easy” adhesive. The above peel strength P1 ismeasured at a temperature T1 in the range between (Ta−10) and (Ta−4),while the peel strength P2 is measured at a temperature in the rangebetween (Ta+4) and (Ta+10).

[0007] That is, U.S. Pat. No. 5,412,035 teaches that thethermal-peeling-easy adhesive, which is be easily peeled off during orafter the use once the adhesive is finally adhered to the adherent, canbe produced through the optimum combination of the polymericpressure-sensitive adhesive component and the crystalline polymer whichis intimately mixed with the polymeric pressure-sensitive adhesivecomponent. This US patent specification or the corresponding JP-Apublication does not disclose that polycaprolactone may be used as acrystalline polymer.

[0008] JP-A-2000-119624 discloses a heat-activation adhesive comprisinga specific tackifying polymer and a polyester such as polycaprolactone,etc. Using the disclosed adhesive, an article such as an electronic partor a polymer film can be adhered to an adherent by heat pressing(pressing after heating or pressing while heating). Polycaprolactoneefficiently functions as a thermoplastic polymer, and the combination ofpolycaprolactone and the tackifying polymer having good compatibilitywith polycaprolactone can form a heat-activation adhesive havingimproved adhesion strength.

[0009] It is essential for the disclosed tackifying polymer to have twofunctional groups, that is, a hydroxyl group and a phenyl group, in themolecule, and the compatibility with polycaprolactone is improved by thefunctions of these functional groups.

[0010] With the above-described thermal-peeling-easy adhesive disclosedin U.S. Pat No. 5,412,035, an adhered article (e.g. an adhesive sheetcomprising a substrate fixed to an adhesive layer containing anadhesive, etc.) can be relatively easily peeled off from an adherent,after it is heated to a specific temperature, usually, a temperaturehigher than the melting point of the crystalline polymer. However, theconventional thermal-peeling-easy adhesives may be insufficient in thefollowing properties, and thus their improvements are necessary:

[0011] (i) The easily peelable state is maintained for a specific periodof time (to prolong the easily peeling time),

[0012] (ii) The adhered article can be peeled off cleanly withoutleaving the adhesive (the polymeric components such as the tackifyingpolymer) on the adherent (to avoid so-called “adhesive leavings”).

[0013] With the conventional adhesives, the thermal-peeling-easy effectis relatively quickly lost when the temperature of the adherent becomeslower than a certain specific temperature. For example, the adhesivesrecover the peel strength of substantially the same level as that priorto heating only within 2 to 3 minutes after heating.

[0014] For instance, when such an adhesive is used as the adhesive layerof an adhesive sheet to be used outdoors (e.g. exterior decorativesheets or reflection sheets for signs, etc.), and the adhesive sheet ispeeled off under relatively low temperature conditions such as winterseasons, the temperature of the adherent is spontaneously lowered to atemperature less than the specific temperature before the entire sheetis peeled off.

[0015] When such as adhesive is used as the adhesive layer of anadhesive sheet having a relatively large area (usually 400 cm² orlarger), the following problem may arise:

[0016] When a certain part of the adherent is heated through theadhesive sheet, and then other part of the adherent is heated, the onceheated certain part starts to cool while the other part is heated. Evenwhen the entire adhesive sheet and the adherent are uniformly heated,one part of the adhesive sheet start to cool while other part of theadhesive sheet is being peeled off.

[0017] When the above adhesive is used to adhere relatively small parts(e.g. electronic parts, etc.) to adherents (e.g. other electronicparts), the following problem may arise:

[0018] When the relatively small articles are peeled off and separatedfrom the adherents, it is efficient to heat a plurality of the parts inan oven or the like, take them out from the oven and then remove eachpart from the adherent. However, if the number of the parts is large,the remaining parts are cooled while some parts are being removed fromthe adherent.

[0019] In any case, when the peelable time is short, the peeling workbecomes difficult.

[0020] Sometimes, the articles such as the adhesive sheets or the partsshould be removed from the adherents, after they are used for arelatively long time (several months or longer). When the articles areremoved from the adherents after the long time use, the adhesive layeris cohesively failed, so that the adhesive layer is often left on theadherent. Such adhesive leavings should be prevented in anyapplications.

[0021] However, none of the above patent specifications and publicationsdisclose any method to solve the above problems (i) and (ii).

[0022] Furthermore, U.S. Pat. No. 5,412,035 does not disclose athermal-peeling-easy adhesive which can be readhered after peeling.Readhesion properties are important properties when the adherents arechanged, or the position of the adhesion on the adherent surface ischanged. In such applications, adhesives, which can be easily readheredonly by pressing, are very advantageous.

[0023] Thus, one object of the present invention is to provide athermal-peeling-easy adhesive composition, (1) which achieves the easilypeelable state by heating it to a specific temperature to decrease thepeel strength to a value smaller than that prior to heating, on adesired occasion after it is adhered to an adherent, (2) which canmaintain such an easily peelable state for a certain period of time, (3)which can be peeled off without leaving the adhesive on the adherent,and (4) which can be easily readhered to an adherent (including otheradherent) after peeling.

SUMMARY OF THE PRESENT INVENTION

[0024] To solve the above problems, the present invention provides anadhesive composition comprising (I) a tackifying polymer and (U) acrystalline polymer, wherein the content of said tackifying polymer isfrom 55 to 95 wt. % based on the whole weight of the composition, andthe content of said crystalline polymer is from 4 to 40 wt. % based onthe whole weight of the composition,

[0025] characterized in that

[0026] said crystalline polymer comprises polycaprolactone, and

[0027] said tackifying polymer comprises a polymer which is compatiblewith said polycaprolactone when the composition is heated to the meltingpoint of said polycaprolactone and is crosslinked.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0028] Similar to the above-described conventional thermal-peeling easyadhesive compositions, the adhesive composition of the present inventioncontains (I) the tackifying polymer and (II) the crystalline polymer,and their contents are 55 to 95 wt. %, and 4 to 40 wt. %, respectively,based on the while weight of the composition.

[0029] The characteristics and properties of the adhesive composition ofthe present invention include:

[0030] (1) that the crystalline polymer comprises polycaprolactone,

[0031] (2) that the tackifying polymer comprises a polymer which iscompatible with polycaprolactone when the composition is heated to themelting point of said polycaprolactone, and

[0032] (3) that the polymer compatible with polycaprolactone iscrosslinked.

[0033] Because of these characteristics, the present invention canprovide the thermal-peeling-easy adhesive composition,

[0034] (a) that can improve the thermal-peeling-easy properties thatachieve the easily peelable state by heating it to a specifictemperature to decrease the peel strength to a value smaller than thatprior to heating, on a desired occasion after it is adhered to anadherent,

[0035] (b) that can maintain such an easily peelable state, and prolongthe easily peelable time to a desired period of time,

[0036] (c) that can improve the removal cleanliness, that is, it can bepeeled off without leaving the adhesive on the adherent, and

[0037] (d) that can improve the readhesion properties, that is, afterpeeling, it can be easily readhered to an adherent by pressing it to theadherent.

[0038] The first property, that is, the inclusion of polycaprolactone asthe crystalline polymer effectively contributes to, in particular, theeffect described in (a) above.

[0039] Polycaprolactone easily melts when it is heated to a temperaturenot lower than its melting point and becomes compatible with thetackifying polymer so that it increases the peeling easiness.Furthermore, since polycaprolactone is a non-tackifying polymer, itincreases the removal cleanliness, and avoids the adhesive leavings onthe adherent surface after peeling.

[0040] The second property contributes to, in particular, the effects(b) and (d) above. That is, since the tackifying polymer is compatiblewith the molten polycaprolactone, the crystallization(recrystallization) of polycaprolactone can be retarded after heating.In addition, the melting and recrystallization of polycaprolactone inthe adhesive composition are substantially reversible physical changes(phenomena). Thus, the composition can restore substantially the samestate as that prior to heating, when and after it is readheredsubsequent to the heating of the composition. Furthermore, thecomposition can be again peeled off after it is readhered.

[0041] As will be explained in detail later, the adhesion composition ofthe present invention can be adhered by pressing even at roomtemperature (about 15 to 30° C.). In addition, it can be adhered by heatpressing, when the content of the tackifying polymer is relativelysmall, or it is adhered to an adherent to which the adhesion of anadhesive is difficult. Furthermore, the adhesive composition may bereadhered to the adherent while it is not cooled after the thermalpeeling.

[0042] The third property contributes to, in particular, the effectdescribed in (c) above. Since the cohesive force is effectivelyincreased by the crosslinking, the leaving of the tackifying polymer onthe adherent can be avoided when the adhesive composition is peeled off.In the peeling-easy state, polycaprolactone is intimately mixed with thecrosslinked tackifying polymer. Thus, the adhesive leavings of theentire adhesive composition can be prevented without requiring thecrosslinking of polycaprolactone, which will prevent the melting andrecrystallization of polycaprolactone.

[0043] From such a viewpoint, it is preferable to use a crosslinkingcomponent which does not substantially chemically react withpolycaprolactone, when the adhesive composition further comprises acrosslinking component (crosslinking agent). Since polycaprolactoneusually has hydroxyl groups at both polymer ends, the tackifying polymerpreferably has a functional group other than hydroxyl groups (e.g. acarboxyl group, etc.) as a crosslinkable functional group reactive withthe crosslinking component. Such a functional group and such acrosslinking composition will be explained below.

[0044] To achieve the above effects of the present invention, thecontent of the tackifying polymer is in the range between 55 and 95 wt.% based on the whole weight of the adhesive composition. When thecontent of the tackifying polymer is less than 55 wt. %, in particular,the effect (d) may not be attained. When this content exceeds 95 wt. %,in particular, the effects (a) and (b) may not be attained.

[0045] Meanwhile, the content of the crystalline polymer is in the rangebetween 4 and 40 wt. % based on the whole weight of the adhesivecomposition. Generally, when the content of the crystalline polymer isless than 4 wt. %, in particular, the effects (a) through (c) may not beattained. Furthermore, when this content exceeds 40 wt. %, inparticular, the effect (d) may not be attained.

[0046] From such viewpoints, the content of the tackifying polymer ispreferably from 60 to 94 wt. %, more preferably from 65 to 90 wt. %, andthe content of the crystalline polymer is preferably from 5 to 35 wt. %,more preferably from 9 to 32 wt. %.

[0047] Polycaprolactone

[0048] Polycaprolactone used in the present invention may be anypolycaprolactone, insofar as it is substantially non-tacky at roomtemperature (about 25° C.), and it has crystallinity such that it can bemolten by heating. Polycaprolactone may be (i) one obtained bypolymerizing a starting material containing caprolactone, or (ii) onehaving recurring units obtained by ring opening polymerization ofcaprolactone.

[0049] In the case of an adhesive composition comprising the tackifyingpolymer and polycaprolactone, it is possible to substantially suppressthe tackiness at room temperature by the crystallization ofpolycaprolactone. However, the composition can exhibit the desired levelof adhesion force by pressing the adhesive composition to the adherentat room temperature or an elevated temperature. When the higher adhesionforce is required, the composition may be heat pressed.

[0050] The melting point of polycaprolactone may depend on its molecularweight, and is usually in the range between 30 and 70° C., preferably inthe range between 35 and 65° C., in particular in the range between 40and 60° C.

[0051] The molecular weight of polycaprolactone may not be limitedinsofar as the adhesive composition can exhibit the desired adhesionforce. In general, its weight average molecular weight is from 1,000 to100,000, preferably from 2,000 to 50,000, in particular from 3,000 to40,000. When the molecular weight is too small, the adhesion force tendsto decrease. When the molecular weight is too large, the compatibilityof polycaprolactone with the tackifying polymer may deteriorate, so thatthe above-described intended effects may not be attained.

[0052] The adhesive composition of the present invention may contain acrystalline polymer other than polycaprolactone, insofar as the effectsof the present invention may not be impaired.

[0053] In addition, a urethane-modified polymer, that is chain-extendedby the reaction of a diisocyanate compound and polycaprolactone, may beused, insofar as the effects of the present invention are not impaired.

[0054] Tackifying Polymer

[0055] The tackifying polymer used in the present invention is a polymerwhich is tacky at room temperature (about 25° C.), and includes acrosslinkable polymer which is compatible with the above-describedpolycaprolactone when it is heated to a temperature not lower than themelting point of polycaprolactone. The tackifying polymer should becrosslinked, when the adhesive composition is used as athermal-peeling-easy adhesive according to the present invention.

[0056] Whether the tackifying polymer is compatibilized withpolycaprolactone when heated to a temperature not lower than the meltingpoint of polycaprolactone can be determined with the clarity of theadhesive composition, that is, the change (decrease) of haze. Forexample, the transparency of a film adhesive (a film-form adhesive)consisting of the adhesive composition of the present invention andhaving a thickness of 30 to 60 μm is compared between the heated stateat a temperature not lower than the melting point of polycaprolactoneand the state at room temperature (about 25° C.) or less. At roomtemperature, polycaprolactone usually forms fine crystals and dispersedin the matrix comprising the tackifying polymer. Thus, the film adhesivehas relatively high transparency, and the haze measured with a colordifference meter is 5% or more (usually 20% or less). Whenpolycaprolactone is molten and compatibilized with the tackifyingpolymer, the film adhesive becomes substantially transparent. Ifpolycaprolactone is molten but is not compatibilized with the tackifyingpolymer, the haze does not substantially change. In such a case, thesmaller haze means better compatibility. Accordingly, the haze of thefilm adhesive measured with the color difference meter is preferably 3%or less, more preferably 2% or less, when polycaprolactone and thetackifying polymer are in the compatibilized state.

[0057] The compatibility of polycaprolactone and the tackifying polymercan be simply judged by the transparency of a solution containing thetwo polymers. That is, one prerequisite for the good compatibilitybetween the tackifying polymer and polycaprolactone is that atransparent mixture forms, when the first solution containing thedissolved tackifying polymer and the second solution containing thedissolved polycaprolactone are mixed.

[0058] The compatibility of the two polymers can be confirmed bychecking the transmission of polarized light with a polarizationmicroscope. As is well known, when the polarization axes of a pair ofpolarizing plates are crossed at right angles, no light passes, so thatthe view becomes substantially dark.

[0059] A pair of polarizing plates, that are arranged so that theirpolarization axes are crossed at right angles, are observed withinserting the film adhesive made of the adhesive composition of thepresent invention between them. At room temperature, the fine crystalsof polycaprolactone rotate the polarizing plane of the light enteringthe film adhesive, and thus the light is allowed to pass through theboth polarization plates. Since the directions of the crystal axes areusually random, polycaprolactone contains the crystals which rotate thepolarization plane of the light just 90 degrees to allow the light topass through the both polarization plates, and also the crystals thathardly allow the light to pass through the polarization plates. As thefine crystals of polycaprolactone are smaller and better dispersed, theyhave the larger compatibility with the tackifying polymer. Accordingly,as the compatibility of the both polymers increases, the crystal sizedecreases, and thus the entire film in the field of view of themicroscope (100 to 200 times magnification) is observed faintly bright.When the compatibility of the polymers is low, the crystal size becomeslarge, and thus the crystals can be observed as bright spots that aresprinkled on the dark background. When polycaprolactone is molten andbecomes compatible with the tackifying polymer, the polymer mixturecontained in the film adhesive is optically isotropic, and is darkerthan at room temperature.

[0060] Examples of the tackifying polymer include acrylic polymers,nitrile-butadiene copolymers (NBR, etc.), styrenebutadiene copolymers(SBR, etc.), polyurethane, silicone polymers, etc. The tackifyingpolymers may be used singly or in admixture of two or more.

[0061] To increase the compatibility with polycaprolactone, thetackifying polymer preferably has (a) a hydroxyl group and (b) a phenylgroup as essential functional groups in the molecule.

[0062] To impart the crosslinking properties, the tackifying polymerpreferably has (c) a crosslinkable functional group as a furtheressential functional group. The crosslinkable functional group means afunctional group which can participate in the crosslinking reaction whenthe polymer is heated or irradiated with electromagnetic waves(including UV ray) or electron beams, and usually a functional groupwhich reacts with the crosslinking component contained in the adhesivecomposition. Alternatively, the molecules of the tackifying polymer maybe directly crosslinked through the crosslinkable functional groups. Thedetails of the crosslinkable functional groups and the crosslinkingcomponent will be explained in detail.

[0063] The above polymer having the essential functional groups in themolecule may be prepared by polymerizing a starting monomer mixturecontaining a monomer having a hydroxyl group in the molecule, a monomerhaving a phenyl group in the molecule and a monomer having acrosslinkable functional group in the molecule. Alternatively, thecarboxyl groups in the polymer may be converted to hydroxyl groups andphenyl groups.

[0064] One preferable example of the acrylic polymer used in the presentinvention is explained.

[0065] Such a polymer is an acrylic polymer prepared by polymerizing amonomer mixture containing (A) a (meth)acrylic monomer having a hydroxylgroup in the molecule, (B) a (meth)acrylic monomer having a phenyl groupin the molecule, (C) a (meth)acrylic monomer having a crosslinkablefunctional group in the molecule, and (D) an alkyl acrylate having 4 to10 carbon atoms in the alkyl group. Such a polymer may be prepared byany conventional polymerization method such as solution polymerization.

[0066] Examples of the monomer (A) include 2-hydroxyethyl acrylate,2-hydroxypropyl acrylate, 2-hydroxymethyl acrylate, 2-hydroxyethylmethacrylate, 2-hydroxypropyl methacrylate, 2-hydroxy-3-pheoxypropylacrylate, etc.

[0067] As the monomer (A), one having both a hydroxyl group and a phenylgroup in the molecule such as 2-hydroxy-3pheoxypropyl acrylate ispreferable. With such a monomer (A), the compatibility of the tackifyingpolymer with polycarolactone is effectively improved.

[0068] Examples of the monomer (B) include those having a phenoxy groupas a phenyl group such as phenoxyethyl acrylate, phenoxypropyl acrylate,etc.

[0069] Examples of the monomer (D) include n-butyl acrylate, isobutylacrylate, isooctyl acrylate, 2-ethylhexyl acrylate, etc. The monomer (D)has none of the above functional groups (a), (b) and (c).

[0070] As the monomer (C), unsaturated carboxylic acids such as(meth)acrylic acid; compounds having a thermally crosslikable group suchas epoxy group-containing (meth)acrylic monomers (e.g. glycidyl(meth)acrylate, etc.) and the like can be used. Furthermore,(meth)acrylic monomers having a unsaturated double bond and aphotocrosslinkable functional group in the molecule are used to obtain aphotocrosslinkable tackifying polymer. In this case, the monomer (C)having both a thermally crosslinkable group and a photocrosslinkablegroup may be used so that the tackifying polymer can be crosslinked withheat and light.

[0071] The total amount of the recurring units derived from thecomponents (A) and (B) in the whole tackifying polymer is usually from40 to 90 wt. %, preferably from 41 to 85 wt. %, in particular from 42 to80 wt. %.

[0072] When the total amount of the recurring units derived from thecomponents (A) and (B) is too low, the compatibility of the tackifyingpolymer with polycarolactone tends to decrease. When this total amountis too high and in turn the amount of the component having otherfunctional group is too low, the crosslinkability or tackiness tends todecrease, and the desired properties may not be effectively improved.For example, the decrease of the crosslinkability may lead to thedeterioration of the effect to prevent adhesive leavings. The decreaseof the tackiness of the tackifying polymer may deteriorate thepress-adhesion property, that is, the property to adhere an article toan adherent by pressing. When the press-adhesion property is regardedimportant, the total amount of the recurring units derived from thecomponents (A) and (B) is preferably in the range between 42 and 60 wt.%.

[0073] The proportion of the recurring units derived from the monomer(B) in the whole recurring units of the tackifying polymer is usually atleast 0.5 mole %, preferably at least 1 mole %, in particular from 5 to25 mole %. When the proportion of the recurring units derived from themonomer (B) is too low, the compatibility of the tackifying polymer withpolycaprolactone tends to decrease. When this proportion is too high,the press-adhesion property may deteriorate.

[0074] The proportion of the recurring units derived from the monomer(C) in the whole recurring units of the tackifying polymer is usuallyfrom 0.5 to 15 wt. %, preferably from 0.7 to 10 wt. %, in particularfrom 1 to 7 wt. %.

[0075] In addition to the polymers having the above essential functionalgroups, that is, the hydroxyl group, the phenyl group and thecrosslinkable functional group, the tackifying polymer may be used incombination with a polymer that does not have such functional groups.However, the proportion of the polymers having the essential functionalgroups is at least 50 wt. %, preferably at least 60 wt. %, in particularat least 70 wt. %.

[0076] Other preferable example of the acrylic polymer to be used in thepresent invention is an acrylic polymer prepared by polymerizing (i) analkyl acrylate mixture of 2-ethylhexyl acrylate, butyl acrylate andmethyl acrylate, and (ii) acrylic acid or methacrylic acid. Such anacrylic polymer also has good compatibility with polycaprolactone. Whensuch an acrylic polymer is used as a tackifying polymer, the adhesivecomposition preferably contain a crosslinking component reactive withthe carboxyl group of (meth)acrylic acid (ii).

[0077] The proportion (weight percentage) of the recurring units derivedfrom the monomers (i) in the whole recurring units of such a tackifyingpolymer is usually from 70 to 99 wt. %, preferably from 80 to 97 wt. %,in particular from 85 to 95 wt. %.

[0078] The proportion (weight percentage) of the recurring units derivedfrom the monomers (ii) is usually from 0.5 to 30 wt. %, preferably from2 to 20 wt. %, in particular from 4 to 15 wt. %.

[0079] The molecular weight of the tackifying polymer used in thepresent invention is not limited insofar as the adhesive composition canexert the desired adhesion force. In general, the weight averagemolecular weight of the tackifying polymer is from 10,000 to 1,000,000.

[0080] A tackifier may be used together with the tackifying polymer,like in the case of conventional pressure-sensitive adhesives.

[0081] Crosslinkable Functional Groups

[0082] The crosslinkable functional group of the tackifying polymer ispreferably a functional group other than the hydroxyl group (a), whichis reactive with the thermally crosslinking component. Preferably, thetackifying polymer has at least one of a carboxyl group and an epoxygroup as the crosslinkable functional group, while it may have both ofthem.

[0083] In such a case, a preferred thermally crosslinking component iscompound having at least two crosslinkable functional groups reactivewith the carboxyl group and/or the epoxy groups of the tackifyingpolymer. Such a compound is usually a monomer or an oligomer.

[0084] Examples of the combination of the crosslinkable functional groupand the thermally crosslikable component are as follows:

[0085] (1) When the crosslinkable functional group is the epoxy group,the thermally crosslinking component is preferably a bisamidecrosslinking agent or an epoxy resin.

[0086] (2) When the crosslinkable functional group is the epoxy resin,the thermally crosslinking component is preferably a rosin having acarboxyl group in the molecule (carboxyrosin).

[0087] Either the carboxyl group or the epoxy group can effect thethermal crosslinking without deteriorating the compatibilizing effectsof the hydroxyl group and the phenyl group of the tackifying polymerwith polycaprolactone. In addition, it is easy to achieve the sufficientcrosslinking degree for the increase of the heat stability and theeffect to prevent the adhesive leavings by the thermal crosslinkingreaction.

[0088] In the case of the above (1), carboxyrosin may be used together,while in the case of the above (2), the epoxy resin and/or the bisamidecrosslinking agent may be used together.

[0089] Crosslinking Components

[0090] The epoxy resin reacts with the carboxyl group of the tackifyingpolymer and functions to thermally crosslink the tackifying polymer.Examples of the epoxy resin include bisphenol A epoxy resins, bisphenolF epoxy resins, cresol-novolak epoxy resins, phenol-novolak epoxyresins, etc.

[0091] The epoxy equivalent of the epoxy resin is usually from 70 to400, preferably from 80 to 300.

[0092] As the bisamide crosslinking agent, bisaziridine derivatives ofdibasic acids such as isophthaloyl bis(2-methylaziridine) may be used.The bisamide crosslinking agent is particularly preferable since it canreact with the tackifying polymer having the carboxyl group at roomtemperature and easily achieves the sufficient crosslinking degree.

[0093] When the tackifying polymer has the epoxy group in the molecule,preferable crosslinking components are carboxyrosins. The carboxyrosinshave the carboxyl group in the molecule and react with the tackifyingpolymer. Thus, they function to thermally crosslink the tackifyingpolymer. As the carboxyrosin, gum rosin, wood rosin, tall oil rosin, andtheir chemically modified products (e.g. polymer rosin, etc.) may beused.

[0094] The carboxyrosins may be used singly or in admixture of two ormore. Rosins having substantially no carboxyl group may be used togetherinsofar as the effects of the present invention are not impaired.

[0095] When the above-described crosslinking component is used, theproportion of the crosslinking component in the whole adhesivecomposition (whole weight) is usually from 1 to 30 wt. %, preferablyfrom 2 to 25 wt. %.

[0096] A reaction accelerator for the crosslinking component such as anepoxy resin may be compounded in the adhesive composition. Thereby, thethermal crosslinking conditions may be made mild.

[0097] The crosslinking components are not limited to those describedabove, and any other crosslinking component may be selected according tothe kinds of the crosslinkable functional groups, crosslinkingconditions, etc.

[0098] Preparation of Adhesive Composition

[0099] The adhesive composition of the present invention may be preparedby uniformly mixing the raw materials by a conventional mixing method.For example, the tackifying polymer, polycaprolactone, the crosslinkingagent, the solvent, etc. are mixed with a mixing apparatus such as ahomomixer, a planetary mixer, etc. to homogeneously dissolve or dispersethe materials to obtain a liquid composition.

[0100] Such a liquid composition can be prepared in the form of aprecursor solution containing the tackifying polymer andpolycaprolactone by mixing the first solution comprising the dissolvedtackifying polymer and the second solution comprising dissolvedpolycaprolactone. Then, the precursor solution is dried to obtain theadhesive composition consisting of the dried material of the precursorsolution. In such a way, it is possible to form a specific morphology(interlinked structure) of crystalline polycaprolactone and thetackifying polymer having good compatibility with polycaprolactone, andthus the above-described properties (non-tackiness at room temperatureand high adhesion force) can be most effectively achieved. When thecrosslinking component is added, the third solution comprising thecrosslinking component is added to the precursor solution.

[0101] For example, the above-prepared precursor solution is applied ona substrate and dried to form a film adhesive (or an adhesive layer)comprising the adhesive composition. As application means, conventionaltools such as knife coaters, roll coaters, die coaters, bar coaters,etc. may be used.

[0102] As the substrate, a releasable substrate such as a liner, asubstrate for an adhesive sheet (support), etc. may be used.

[0103] The drying of the applied precursor solution is usually carriedout at a temperature of 60 to 180° C. The drying time is usually fromseveral ten seconds to several minutes.

[0104] The thickness of the film adhesive is usually from 5 to 1,000 μm,preferably from 10 to 500 μm, in particular from 15 to 100 μm.

[0105] When the film adhesive according to the present invention is usedwith adhering it to a suitable adherent, the adhesion is completed bypressing the film adhesive to the adherent under a pressure of 1 to 50kg/cm² (about 0.1 to 4.9 MPa) after laminating the film adhesive on theadherent. In the course of pressing, the initial adhesion force may beincreased by heating and cooling (spontaneous cooling). In this case,the heating conditions are not limited. In general, the heatingtemperature is from 60 to 120°, and the heating time is from 30 secondsto 5 minutes.

[0106] The film adhesive according to the present invention may be usedas the adhesive layer of an adhesive sheet.

[0107] Thermally Easy Peelable Adhered Structure

[0108] The adhesive composition of the present invention has thethermally easy-peeling properties. Thus, it can form an adhesivestructure having the following construction:

[0109] A thermally easy peelable adhered structure comprising (1) anadherent, (2) an adhesive layer consisting of the adhesive compositionof the present invention adhered to the surface of the adherent, and (3)an article fixed to the adhesive layer. In this structure, at anydesired time, the article can be peeled off from the adherent by heatingthe structure to the specific temperature without leaving any adhesiveon the adherent. Those having coated surfaces as the adhering surfacesmay be used.

[0110] Examples of the adherent to be used include adherents having thesurfaces (adhering surfaces) made of (1) metals such as aluminum,stainless steel, copper, zinc-plated steel plate, etc., (2) resins suchas polyimide, acrylic resins, polyurethane, melamine resins, epoxyresins, vinyl chloride resins, etc., and (3) inorganic oxide materialssuch as ceramics, etc.

[0111] Examples of the articles include parts such as electronic parts,machine parts, optical parts, etc., and substrates for theabove-described adhesive sheets, and so on.

[0112] In the thermally easy peelable adhered structure of the presentinvention, the combination of the adhesive layer and the substrate isusually the thermally easy-peeling adhesive sheet (including an adhesivetape and a film adhesive). Such a thermally easy-peeling adhesive sheetcomprises a sheet-film substrate having a surface and a back surfaceopposing the surface, and an adhesive layer consisting of theabove-described adhesive composition fixed to the back surface of thesubstrate, and is used with being adhered to the surface of the adherentthrough the adhesive layer.

[0113] The adhesive layer comprising the adhesive composition hassubstantially no tackiness at room temperature. Thus, it has good easyof slide, and therefore the positioning of the adhesive sheet inrelation to the adherent is very easy in the adhering process. In themeantime, after the positioning, the adhesive sheet can be fixed to theadherent by simply pressing or by heat pressing followed by cooling. Theadhesive sheet, which can be easily positioned and press adhered, ispreferably used as an adhesive sheet having a relatively large area (ingeneral, at least 400 cm²) (e.g. interior or exterior decorativesheets), or relatively, large-size retroreflective sheets for marking.Furthermore, the adhesive sheet of the present invention can be used asan application tape or film, since it can be peeled off at any time byheating without the adhesive leavings.

[0114] As the substrate of the adhesive sheet, any conventionally usedsubstrate (support), for example, paper, metal films, polymer films,etc. may be used.

[0115] Examples of the polymers of the polymer films include polyimide,polyvinyl chloride, acrylic polymers, polyesters (e.g. polyethyleneterephthalate, etc.), polyurethane, polyolefin polymers (includingethylene copolymers), and so on.

[0116] The substrate may be one allowing visible light or UV rays totransmit there-through, or one reflecting light like a retroreflectivesheet. Also, the substrate may be colored or decorated by printing, etc.In such cases, the adhesive sheet having the adhesive layer of theadhesive composition of the present invention is useful as a decorativesheet or a marking film.

[0117] The adhering surface of the adhesive layer is usually protectedwith a liner. The liner may be made of a paper sheet, a plastic film, ora laminate of a paper sheet and a plastic film.

[0118] In the case of the thermally easy-peeling adhesive sheet, theadhesion between the adhesive layer and the substrate is preferably asstrong as possible. If the adhesion between the adhesive layer and thesubstrate is weak, a part or whole of the adhesive layer is left on theadhering surface of the adherent, when the adhesive sheet is peeled offfrom the adherent. To increase the adhesion between the adhesive layerand the substrate, a primer is applied to the surface of the substrateon which the adhesive layer is placed.

[0119] Preferably, the polymer constituting the substrate film comprisesat least one of polyurethane and an ethyleneacrylic acid copolymer,since such a polymer film can be strongly adhered to the adhesive layerof the adhesive composition of the present invention without the use ofa primer.

[0120] The thickness of the substrate is usually from 5 to 500 μm,preferably from 10 to 300 μm, and the thickness of the adhesive layer isusually from 10 to 500 μm, preferably from 15 to 300 μm, in particularfrom 20 to 100 μm.

[0121] Furthermore, the adhesive layer may contain any one ofconventional additives, insofar as the effects of the present inventionare not impaired. Examples of such additives include viscositymodifiers, defoaming agents, leveling agents, UV ray absorbers,antioxidants, pigments, fungicides, elastic fine particles of tacky ornon-tacky rubbery polymers, tackifiers, catalysts to accelerate thecrosslinking reaction, etc.

[0122] When the adhesive sheet is thermally peeled off, it is usuallyheated at a temperature of 60 to 120° C. for 30 seconds to 5 minutes.Since the adhesive layer of the adhesive composition according to thepresent invention is used, the easy-peeling time lasts at least 5minutes after the heating to peel the adhesive sheet.

[0123] Preferably, the easy-peeling condition may be maintained evenwhen the adherent and the adhesive layer are cooled to room temperature(about 25° C.), but the adhesion force (peel strength) again increasesafter 15 minutes from cooling to finish the readhesion.

[0124] In the course of the thermal peeling, the adherent and/or theadhesive sheet may be heated with a heating apparatus such as an iron, adrier, an IR (far IR) lamp, etc. When the substrate comprises a metalfoil, the adhesive sheet may be heated by an electromagnetic inductionheating method. Furthermore, the adherent and/or the adhesive sheet maybe heated with a heat transfer medium such as a liquid or steam.

[0125] The peel strength of the adhesive sheet prior to heating forpeeling off is usually at least 10 N/25 mm, preferably from 12 to 40N/25 mm, in particular from 13 to 22 N/25 mm, when it is measured by the180 degrees peeling test at a peeling rate of 300 mm/min. When the peelstrength prior to the thermal peeling process is too low, the adhesivesheet may not be used in the same way as conventional adhesive sheets.When the peel strength prior to the thermal peeling process is too high,the peel strength in the thermal peeling process is too high, and thusthe easy-peeling properties may not be improved.

[0126] An optimum range of the peel strength in the thermal peelingprocess can be suitably selected according to the mechanical strength ofthe substrate (e.g. elastic modulus, elongation at break, etc.), peelingconditions (e.g. peeling rate), and so on. From the viewpoint of quickpeeling, the peel strength is preferably 15 N/25 mm or less, when it ismeasured by the 180 degrees peeling test at a peeling rate of 300mm/min.

[0127] As can be seen from the above explanations, the above-describedadhesive sheet (1) achieves the easily peelable state by heating it to aspecific temperature to decrease the peel strength to a value smallerthan that prior to heating, on a desired occasion after it is adhered toan adherent, (2) can maintain such an easily peelable state for acertain period of time, (3) can be peeled off without leaving theadhesive on the adherent, and (4) can be easily readhered to an adherent(including other adherent) after peeling.

EXAMPLES Example 1

[0128] The adhesive composition of this Example was prepared as follows:

[0129] A composition solution was prepared by mixing a solutioncontaining a tackifying polymer, which was produced as described below(solvent: a mixture of 85 wt. % of ethyl acetate and 15 wt. % of methylethyl ketone; nonvolatile concentration: 30 wt. %), a solution ofunder-mentioned polycaprolactone in toluene (nonvolatile concentration:35 wt. %) and isophthaloyl bis(2-methylaziridine) as a thermallycrosslinking bisamide. The proportions of these components were suchthat tackifying polymer:polycaprolactone: crosslinkingcomponent=70:30:0.2 (weight ratio of the nonvolatiles). The obtainedmixed solution was transparent.

[0130] The composition solution was applied on a release surface of apaper liner having a silicone-coated release surface and dried in anoven at 95° C. for 5 minutes to form an adhesive layer (film adhesive)consisting of the adhesive composition of this Example on the liner.

[0131] This adhesive layer and a polyurethane film having a thickness of33 μm as a substrate were adhered each other by pressing to obtain theadhesive sheet of this Example.

[0132] The thickness of the adhesive layer was 30 μm.

[0133] The used polycaprolactone was PLACCEL® HIP (available from DaicelChemical Industries, Ltd.; weight average molecular weight Mw=25,000,melting point Tm=60° C.), which is represented by “PCL1” in Table 1.

[0134] The tackifying polymer was a copolymer prepared by solutionpolymerizing the monomer mixture containing (A) a monomer having aphenoxy group and a hydroxyl group in the molecule(2-hydroxy-3-phenoxypropyl acrylate (represented by HPPA; ARONICS®M-5700 available from TOAGOSEI Co., Ltd.), (B) a monomer having aphenoxy group in the molecule (phenoxyethyl acrylate (represented byPEA; BISCOAT® #192 available from OSAKA YUKIKAGAKU KOGYOKABUSHIKIKAISHA), (C). acrylic acid (represented by AA; available fromWako Pure Chemical Industries, Ltd.), and (D) n-butyl acrylate(represented by BA) available from TOAGOSEI Co., Ltd.). The ratio ofPEA:HPPA:BA:AA in the tackifying polymer (TAP1 in Table 1) was30:15:50:5.

Example 2

[0135] An adhesive sheet of this Example was produced in the same manneras in Example 1 except that a tackifying polymer having the followingcomposition (TAP2 in Table 1) was used.

[0136] The ratio of PEA:HPPA:EHA:AA in the tackifying polymer was30:15:50:5, in which EHA represents 2-ethylhexyl acrylate.

Example 3

[0137] An adhesive sheet of this Example was produced in the same manneras in Example 1 except that PLACCEL® 220N (available from DaicelChemical Industries, Ltd.; weight average molecular weight Mw=3,800,melting point Tm=50° C.), which is represented by PCL 2 in Table 1, wasused as polycaprolactone.

Example 4

[0138] An adhesive sheet of this Example was produced in the same manneras in Example 3 except that TAP2 used in Example 2 was used as atackifying polymer.

Comparative Examples 1 and 2

[0139] Adhesive sheets used in Comparative Examples 1 and 2 wereSCOTCHCAL® JS 1000A (available from 3M, USA) and SCOTCHCAL® 3650PR(available from 3M, USA), respectively. These adhesive sheets comprisean adhesive layer containing no polycaprolactone.

[0140] The properties of the adhesive sheets produced or used in theabove examples were evaluated as follows:

[0141] Evaluation Methods

[0142] Peel Strength

[0143] A test piece (150 mm×25 mm) formed from the adhesive sheet ofeach example was press adhered to an adherent (a stoved melamine coatedplate available from NTP) at 20° C. according to JIS Z 0237 8.2.3.

[0144] “Ordinary State 1” means a 180 degree peel strength measured withTENSILON at a peeling rate of 300 mm/min. along the lengthwise directionafter maintaining the sample at 20° C. for 48 hours from the adhesion.

[0145] “Heat Peel” means a peel strength measured by the same method asabove after heating the adhered test piece up to about 100° C. byblowing hot air using an industrial drier against the substrate surfaceof the test piece, and then spontaneously cooling it for 5 minutes.

[0146] The percentage in the brackets in Table 1 is a retentionpercentage of the peel strength after spontaneous cooling in comparisonwith the peel strength of Ordinary State 1.

[0147] “Ordinary state 2” means a peel strength measured by the samemethod as above after heating the adhered test piece in the same way asin the evaluation of “Heat Peel”, and then spontaneously cooling it for15 minutes.

[0148] Slidability

[0149] At a temperature of 20° C., a test piece was placed on the aboveadherent so that the adhesion surface of the adhesive layer was incontact with the adhesion surface of the adherent. Then, whether thetest piece could be easily moved horizontally or not was evaluated withthe hand feeling. When the test piece could be slid with substantiallyno resistance, it was ranked “Good (symbol: O)”, while when the slidingwas difficult due to the high resistance caused by adhesion to theadherent, it was ranked “No good (symbol: X)”.

[0150] Adhesive Leavings

[0151] The surface of the adherent was visually evaluated after the peelstrength test in the heat peel mode. When no adhesive leavings wereobserved, it was ranked “Good (symbol: O)”, when the adhesive leavingswere partially observed, it was ranked “Normal (symbol: Δ)”, and whenthe adhesive leavings were observed over the whole surface, it wasranked “No good (symbol: X)”.

[0152] The results are shown in Table 1. TABLE 1 Tackifying Peelstrength (N/25 mm) polymer/ Ordi- Ordi- polycapro- Slida- nary Heat naryAdhesive lactone bility state 1 peel state 2 leavings Ex. 1 TAP1/PCL1 ◯19 8 (42%) 22 ◯ Ex. 2 TAP2/PCT1 ◯ 16 9 (56%) 19 ◯ Ex. 3 TAP1/PCL2 ◯ 18 4(22%) 17 ◯ Ex. 4 TAP2/PCT2 ◯ 18 5 (27%) 17 ◯ Ex. 5 TAP3/PCT2 — 17 9(53%) 19 ◯ Ex. 6 TAP3/PCL2 — 20 5 (26%) 19 ◯ Ex. 7 TAP1/PCL2 — 15 10(65%)  — ◯ (90:10) Ex. 8 TAP1/PCL2 — 14 9 (64%) — ◯ (80:20) C.E. 1 No X29 30 (103%) 31 Δ polycapro- lactone C.E. 2 No X 23 23 (100%) 24 ◯polycapro- lactone

[0153] From the above results, the following conclusions can be reached:

[0154] In Examples 1 to 4, the slidability was good, and the highadhesion force (peel strength) of at least 16 N/25 mm was attained after48 hours from the press adhesion. Furthermore, the adhesive leaving wasfew after peeling.

[0155] In Examples 1 and 2, the adhesion force decreased to about 50%after the heat peel process, and the adhesive tapes could maintain theeasy peeling state for at least 5 minutes. In the meantime, after 15minutes elapsed, the adhesion force recovered the level prior toheating. In Examples 3 and 4, the adhesion force decreased to about 30%or less after the heat peel process, and the adhesive tapes couldmaintain the easy peeling state for at least 5 minutes. In the meantime,after 15 minutes elapsed, the adhesion force recovered the level priorto heating.

[0156] In Comparative Example 2, the evaluation of the adhesive leavingsafter peeling was better than Comparative Example 1. However, neitherComparative Example 1 nor 2 could achieve the thermally easy-peelingproperties.

Example 5

[0157] An adhesive sheet of this Example was produced in the same manneras in Example 3 except that an adhesive AST 8167 (available from NipponShokubai Co., Ltd.; represented by TAP3 in Table 1) was used as asolution containing tackifying polymer. In the used tackifying polymer,the ratio of EHA:BA:MA:AA was 3.8:49.7:10:6.5, in which MA representsmethyl acrylate, and other symbols are the same as described above.

[0158] A test piece (150 mm×25 mm) formed from the adhesive sheet waspress adhered to the same adherent as that used in Example 1, and heatedat 85° C. for 1 minute to complete the adhesion.

[0159] After maintaining the adhered sample at room temperature for 2hours, a 180 degree peel strength was measured with TENSILON at apeeling rate of 300 mm/min. This result is included in the column of“Ordinary State 1” of Table 1.

[0160] After maintaining the adhered sample at room temperature for 2hours, a peel strength was measured by the same method as above afterheating the adhered test piece up to about 100° C. by blowing hot airusing an industrial drier against the substrate surface of the testpiece, and then spontaneously cooling it for 5 minutes. This result isincluded in the column of “Heat Peel” in Table 1.

[0161] “Ordinary state 2” means a peel strength measured by the samemethod as above after heating the adhered test piece in the same way asin the evaluation of “Heat Peel”, and then spontaneously cooling it for15 minutes.

[0162] The results are also shown in Table 1. In addition, theevaluation result of adhesive leavings is also shown in Table 1.

Example 6

[0163] An adhesive sheet of this Example was produced in the same manneras in Example 5 except that, after the formation of the adhesive layeron the liner and the press fixing of the substrate on the adhesivelayer, the adhesive layer was heated at 85° C. for 1 minute and thenaged at 20° C. for 1 hour.

[0164] A test piece (150 mm×25 mm) formed from the adhesive sheet waspress adhered to the same adherent as that used in Example 1, and heatedat 85° C. for 1 minute to complete the adhesion.

[0165] After maintaining the adhered sample at room temperature for 24hours, a 180 degree peel strength was measured with TENSILON at apeeling rate of 300 mm/min. This result is included in the column of“Ordinary State 1” of Table 1.

[0166] After maintaining the adhered sample at room temperature for 24hours, a peel strength was measured by the same method as above afterheating the adhered test piece up to about 100° C. by blowing hot airusing an industrial drier against the substrate surface of the testpiece, and spontaneously cooling it for 1 minute. This result isincluded in the column of “Heat Peel” in Table 1.

[0167] “Ordinary state 2” means a peel strength measured by the samemethod as above after heating the adhered test piece in the same way asin the evaluation of “Heat Peel”, and then spontaneously cooling it for15 minutes.

[0168] The results are also shown in Table 1. In addition, theevaluation result of adhesive leavings is also shown in Table 1.

Example 7

[0169] An adhesive sheet of this Example was produced in the same manneras in Example 3 except that the ratio of the tackifyingpolymer:polycaprolactone:crosslinking component was changed to 90:10:0.2(weight ratio of the nonvolatiles), and then it was evaluated by thesame manners as in Example 1. The results of the evaluations are shownin Table 1.

Example 8

[0170] An adhesive sheet of this Example was produced in the same manneras in Example 3 except that the ratio of the tackifyingpolymer:polycaprolactone:crosslinking component was changed to 80:20:0.2(weight ratio of the nonvolatiles), and then it was evaluated by thesame manners as in Example 1. The results of the evaluations are shownin Table 1.

It is claimed:
 1. An adhesive composition comprising (I) a tackifyingpolymer and (II) a crystalline polymer, wherein the content of saidtackifying polymer is from 55 to 95 wt. % based on the whole weight ofthe composition, and the content of said crystalline polymer is from 4to 40 wt. % based on the whole weight of the composition, characterizedin that said crystalline polymer comprises polycaprolactone, and saidtackifying polymer comprises a polymer which is compatible with saidpolycaprolactone when the composition is heated to the melting point ofsaid polycaprolactone and is crosslinked.
 2. The adhesive compositionaccording to claim 1, which further comprises a crosslinking component,wherein said polymer compatible with said polycaprolactone is a polymerhaving (a) a hydroxyl group, (b) a phenyl group and (c) a crosslinkablefunctional group reactive with said crosslinking component.
 3. Theadhesive composition according to claim 2, wherein said polymercompatible with said polycaprolactone is an acrylic polymer obtained bypolymerizing a monomer mixture containing (A) a (meth)acrylic monomerhaving a hydroxyl group in the molecule, (B) a (meth)acrylic monomerhaving a phenyl group in the molecule, (C) a (meth)acrylic monomerhaving said crosslinkable functional group in the molecule, and (D) analkyl acrylate having 4 to 10 carbon atoms in the alkyl group.
 4. Anadhered structure which is thermally peeled with ease, comprising anadherent, an adhesive layer comprising the adhesive compositionaccording to any one of claims 1 to 3, and an article adhered to saidadhesive layer, wherein said article can be peeled off without leavingsaid adhesive on said adherent when it is heated to a specifictemperature on a desired occasion.